The race before Iowa's Republican caucuses has narrowed to a two-person contest between former governors Mike Huckabee of Arkansas and Mitt Romney of Massachusetts, with Huckabee now perched atop the field, propelled by a big jump in support among religious women.

The findings, from a new Washington Post-ABC News poll, show how dramatically the wide-open GOP contest has changed over the past few months. Huckabee's support in Iowa has quadrupled since the summer, while former New York mayor Rudolph W. Giuliani and former senator Fred D. Thompson (Tenn.) have lost ground.

Immigration now stands as the top issue for the state's GOP voters. Many Republicans think voter anger about illegal immigration will be a flash point, not only in the race for their party's presidential nomination but also in the general election. The emergence of immigration as a major issue in Iowa, where three in 10 GOP voters call it a top concern, creates an early test of its political potency.

With two weeks to go before the first-in-the-nation caucuses, Huckabee registered the support of 35 percent of likely Republican caucusgoer, just above Romney, the longtime Iowa front-runner, at 27 percent. For the first time in Post-ABC Iowa polling, no other candidate registered in the double digits.

Thompson was backed by 9 percent, down from 15 percent a month ago, and Giuliani was the choice of 8 percent, down five percentage points. Rep. Ron Paul (Tex.) also won 8 percent support, followed by Sen. John McCain (Ariz.) at 6 percent, Rep. Tom Tancredo (Colo.) at 2 percent and Rep. Duncan Hunter (Calif.) at 1 percent.

Huckabee's support in Iowa has risen on a steep curve. In a Post-ABC poll in late July, 8 percent of likely caucusgoers supported him, and last month his backing stood at 24 percent. His increase of 11 percentage points from November came despite a barrage of attacks from Romney and Thompson.

Republican women, particularly those who describe themselves as evangelicals and those who attend church regularly, are the primary force behind Huckabee's recent increases. Women now support him over Romney by an 18-point margin; men divide their votes about evenly between the two.

Over the past month, Huckabee's support among women has doubled, from 22 percent to 45 percent. Female voters have largely abandoned Thompson over the same period, with their support dropping from 14 percent to 3 percent.

The significant growth in Huckabee's support among women may be a sign that his style of social conservatism with a smile is resonating. Four in 10 women said the more they hear about the former Arkansas governor, the more they like him, more than double the proportion saying they like him less as they learn more.

A third of those polled said Huckabee best represents the core values of the Republican Party, whereas 25 percent said Romney does. Huckabee's advantage again stems from women: Fifty percent of women, but only 24 percent of men, said he is the GOP's best standard-bearer.

For all of Huckabee's progress, however, the poll shows a tight race with Romney, whose support has remained stable since the summer.

Romney maintains advantages among voters as the GOP's most electable candidate and the one with the best experience to be president. He also has more committed and more enthusiastic support than he did a month ago.

About half of Romney's backers said they are sure that they will support him at the caucuses, and while 47 percent said there is a chance they could opt for another candidate, that is down from 70 percent a month ago. The percentage saying they are very enthusiastic about voting for him is also up over that time.

The enthusiasm factor will be crucial in deciding the outcome of the caucuses. Iowa's caucuses, unlike traditional primaries, require voters to brave freezing weather and to spend hours debating the merits of the candidates before the balloting.

The new poll numbers may be somewhat dispiriting to Thompson, who has made Iowa a centerpiece of his campaign. He runs a distant third among the conservatives he hoped would be the bedrock of his support, but some believe a strong showing in last week's Des Moines Register debate and the endorsement of Rep. Steve King (R-Iowa) on Monday could spark a resurgence before Jan. 3.

Giuliani, who has led national polls all year, has largely written off Iowa, choosing instead to focus somewhat on the New Hampshire primary on Jan. 8 and casting Florida's Jan. 29 primary as his firewall. The lack of effort in a state that values retail politicking has consequences: His support has dipped below the double digits in Iowa, and the percentages calling him the GOP's strongest leader and most electable candidate have dropped significantly.

Huckabee, by contrast, has made large advances over the past month on four critical attributes. He has made double-digit gains in perceptions that he is the strongest leader in the field, the one with the best chance to win in November (although Romney maintains the edge), the most empathetic and the one who has campaigned the hardest in the state.

While voters have recognized Huckabee's stepped-up efforts in Iowa, Romney, who has focused resources on the state all year, is viewed by more than six in 10 as the candidate who has done the most to win them over.

Huckabee has also broadened his appeal.

His support is still concentrated among Christian conservatives -- 62 percent are evangelical Protestants, 76 percent attend church at least weekly and 88 percent are conservative -- but he is doing somewhat better among more moderate Republicans. Moreover, a month ago, Huckabee was regarded as a single-issue candidate, scoring well as the best able to handle social issues but lagging on other top concerns, including immigration.

Now, as many voters call Huckabee the candidate best able to deal with immigration, taxes, the Iraq war and terrorism as those who favor Romney. Romney maintains an advantage on the economy and Huckabee has a 12-point edge on handling social issues, such as abortion and same-sex civil unions.

Religion remains a key factor in the campaign: Huckabee draws primarily from religious voters, and Romney's Mormon faith may also be an issue. Although a majority of evangelical Protestants said Romney's religion does not make a difference to them, more than a third said they are less likely to support him because he is a Mormon.

Among those who highlight abortion or moral and family values as one of their top two voting issues, Huckabee has a commanding 43-point lead over Romney. Among those citing immigration, 32 percent support Romney and 24 percent back Huckabee, signaling a potential opening for the former Massachusetts governor in the final stretch.

Romney has spent the better part of the past two weeks attacking Huckabee for his record on illegal immigration as governor of Arkansas. In a recent television ad, Romney criticized Huckabee for supporting in-state tuition for the children of illegal immigrants: "On immigration, the choice matters," the ad's narrator intoned.

Huckabee has insisted that Romney is twisting his handling of the issue for political gain, but he has yet to respond directly via television ads. Instead, Huckabee is running a Christmas-themed ad in which he empathizes with voters' fatigue with the campaign.

The poll was conducted by telephone Dec. 13 to 17, among a random sample of 501 Iowa adults likely to attend a Republican caucus on Jan. 3. The results have a margin of sampling error of plus or minus four percentage points. The error margins for subgroups are larger.